I have personally helped pair 150 pairs of males over the past 3 years of rescue between our pigs, and our adopters pigs. We have had one serious fight in that time that resulted in injury to the pigs, and that was in an already rocky pairing that we sent to a foster home to let settle down. They brought a female into the house to "meet" the pigs, and they started fighting over the girl. Until they did that, even that relatively rocky pairing was fine.
Up until last year, I had 3 boars who lived together. Gizmo passed last summer, and now Mogwai and Pica live together.
<-- the forums there have literally thousands of people who keep pairs of boars, as do the forums at guineapigcages.com
I'm actually going this weekend to introduce a rescued boar to another boar this weekend. I like boar/boar pairings, as they tend to settle out quickly.
Keep in mind that most of the literature is extremely out of date and/or authored by breeders who are trying to pair males in extremely small cages and generally in a building full of females who are in heat.
Here are some of the males I've helped pair up:
Peanut and Popcorn (though you can only see Peanut's butt. They're both around 4-5.
Mogwai, Pica, and Gizmo (before Gizmo's passing last year)
Here are Mogwai and Pica, after Gizmo's passing:
3 years ago we took in 30 young males from a backyard breeder. More than half ended up being paired with other males. Some were neutered and paired with females to help them get adopted (as we didn't have females in at the time).
If you check out the reputable guinea pig rescues, most also routinely pair males.
Will some males fight? Yes. But some females will fight, and some females will fight with males. We had a female scale a cage wall to attack a male last year. It's not only a function of gender.